1. Field of the Invention
The present invention relates to a liquid crystal composition and to a LC display device employing the composition. The invention relates more specifically to a LC composition which is preferably used in an active matrix mode display device.
2. Description of the Related Art
Liquid crystal display devices (LCDs) have been used hand-held electronic calculators, digital watches, and so forth, because of a lower driving voltage and smaller electric consumption in comparison with a Brown tube (CRT), and because of probability of miniaturization and reduction in weight and thickness. They have been leading flat-shaped displays. Answering recent necessity for computerization, LCDs have been applied widely to 1 a personal computer, 2 a video camera and a TV, and 3 displays for a motorcar and for outdoor-use. An active matrix addressed LCD (AM-LCD) using thin film transistors (TFT) is expected as a final winner among various LC display modes, since the AM-LCD is believed to enable very fine display (having high contrast) in color. The active matrix addressed display mode employs a TN display mode in which liquid crystal molecules are twisted by 90 degrees to be aligned on each of inner surfaces of a pair of substrates.
A liquid crystal material for display should so properly be selected as to satisfy various characteristics required for an intended use. For example, a liquid crystal material for the above-mentioned 3 displays for a motorcar or for outdoor-use is required to be adaptable to temperature change since the display is used under the environment in which temperature varies greatly. This means that the liquid crystal material is required to exhibit a nematic phase in a widest possible temperature range including a room temperature and a high clearing point particularly, in this case. At present, there has been found no single compound which satisfies those conditions as mentioned above. Hence a number of liquid crystal compounds are mixed together or with non-liquid-crystalline compounds to prepare a liquid crystal composition having necessary characteristics to be applied to an intended use.
In order to extend a nematic range of a LC composition, there should be introduced thereto a liquid crystal compound which has a high clearing point and good miscibility with many other liquid crystal compounds. A LC compound having both a high clearing point and several six-membered rings in chemical structure is known to be employed in order to elevate a clearing point of a resulting composition, however, utility of the composition is limited in this case because there is an increase in viscosity of the composition and also a tendency toward a difficulty in the miscibility at low temperature range.
Further, an AM-LCD requires a LC composition having a so-called high reliability in order to maintain a high contrast in display, in which high reliability means both a high specific resistance of the material and a high voltage holding ratio of the device.
Japanese patent application laid-open No. 58-219137 discloses a liquid crystal composition consisting of 4-(trans-4-alkyloxymethylcyclohexyl)-4'-(trans-4-alkylcyclohexyl)biphenyl compounds and cyano compounds having a terminal CN group(s). However, a LC composition containing a compound having a terminal CN group has a defect of reducing a specific resistance thereof, i.e. decreasing a voltage holding ratio at use temperature. Hence, it is difficult to use the disclosed composition in an AM-LCD.
Although various attempts have been made to liquid crystal materials as described above, it is the present status of art that the LC materials are required to be improved in their various properties.